This invention relates to a device for dispensing a spreadable (i.e. highly viscous) material. The device comprises a tube-like holding element, in which is arranged an internally screwthreaded, non-rotatable piston-like element which carries the spreadable material. The piston-like element is designed for displacement longitudinally between the tube base of the holding element and the dispensing end by an externally operated pedestal-like control wheel provided at end opposite the dispensing end of the holding element. The pedestal-like control wheel comprises an axially supported, screwthreaded spindle that is mounted for rotation in an opening in the tube base and that co-operates with the internal screwthread of the piston-like element.
A device of the type in question has been known for some time, for example from applicants"" DE 195 01 213 C2, which describes an adhesive stick comprising a cylindrical tube of which the open dispensing end is provided with a removable closure cap to stop the adhesive paste accommodated in the tube from drying out when the adhesive stick is not in use. The adhesive paste in the tube is poured into a piston-like element and, together with this piston-like element, is held non-rotatably in the tube and designed for longitudinal displacement in both directions in the tube. The piston-like element is providedxe2x80x94for displacementxe2x80x94with an internally screwthreaded bore that engages a screwthreaded spindle that extends over the entire length of the tube and hence through the adhesive paste. At its end, the spindle merges integrally into a pedestal-like control wheel that resembles a knurled nut and which, at the other end of the tube, is mounted for rotation and projects outwards from the tube at the base thereof. The device is operated by turning the pedestal-like control wheel in one direction or the other while the tube is held firmly, so that the piston-like element and the adhesive paste joined to it are moved in one direction or the other relative to the tube, and adhesive is either dispensed or the adhesive paste is drawn back into the tube.
This known device has been successfully used for some time because it constitutes an easy-to-handle and, at the same time, very stable article. However, it has been found in practice that, unless the device is used properly, the spindle is subjected to such serious torsion if it is turned any further when the piston is in its lowermost position that it breaks off. This results in destruction of the device which thus becomes unusable. The same unwanted effect can also sometimes occur before the filling step of the automated assembly of the device if the tube-like element is moved into the lowermost position by corresponding rotation of the spindle before the device is filled with the spreadable paste.
The problem addressed by the present invention was to improve a device of the type mentioned at the beginning in such a way that torsion of the spindle would be reliably avoided even in the event of incorrect operation and resulting breakage of the spindle.
According to the invention, the external screwthread of the screwthreaded spindle merges in the region adjoining the tube base into a non-screwthreaded region of the spindle that has an external diameter smaller than the internal diameter of the internal screwthread of the piston-like element so that, in the lowermost position of the piston-like element, the spindle is able to slip through relative to the piston-like element if it is rotated any further.
Since the external screwthread of the screwthreaded spindle no longer extends over the entire displacement path of the piston-like element in the device according to the invention, but is missing in the lowermost region so that a screwthread-free region with a smaller external diameter is formed, the spindle is able to slip through relative to the piston-like element if it is rotated any further in the lowermost position of the piston-like element. In this way, the spindle is no longer subjected to torsion and cannot break off, even in the event of incorrect operation, so that the device as a whole remains reliably usable.
In order to prevent the piston-like element or its internal screwthread from moving completely out of contact with the screwthreaded region of the spindle when the piston-like element is in its lowermost position, the length of the screwthread-free region of the spindle is shorter than the length of the internal screwthread of the piston-like element. If then the spindle is turned in the other, opposite direction, the piston-like element can readily be displaced back to the dispensing end because it does not move completely out of contact with the external screwthread of the spindle.
In a particularly preferred embodiment, the height of the screwthread-free region of the spindle is such that, in the lowermost position of the piston-like element, the internal screwthread of the piston-like element remains in contact with the lowermost thread of the spindle. This ensures that the spindle is able to slip through freely relative to the piston-like element and also guarantees that the screwthreads of the piston and spindle do not move completely out of contact.